x86-64: Allow alternative patching in the vDSO

This code is short enough and different enough from the module
loader that it's not worth trying to share anything.

Signed-off-by: Andy Lutomirski <luto@mit.edu>
Link: http://lkml.kernel.org/r/e73112e4381fff29e31b882c2d0856822edaea53.1310563276.git.luto@mit.edu


Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>